Bioimplant

ABSTRACT

Provided is a bioimplant which is capable to inhibit the biofilm formation over a long period of time after an operation. The bioimplant of the present invention comprises a base material of metal, ceramic, or plastic and a thermal spraying film of a calcium phosphate-based material formed at least partially thereon and the silver concentration in the thermal-spray film is 0.05 wt % to 3.00 wt %.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.14/376,183 entitled “Bioimplant” filed on Aug. 1, 2014, the content ofwhich is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an antimicrobial bioimplant.

SUMMARY OF THE INVENTION

Use of bioimplants for treatment of bone injuries/diseases is steadilyincreasing along with expansion of active and elderly populations. Foruse as a bone substitute for broken or removed bones or for use as asupport to assist a weakened bone, the synthetic bone substitute shouldform a strong joint or bone together with natural bones and assure thestructural integrity thereof. A bone can grow into a neighboring tissue,especially when it is a porous tissue similar to the bone. However, inaddition to the growth into porous tissue, the natural bone thus growninto the porous tissue should bind to the bioimplant, forming strongadhesion between them.

An important requirement for fixation of a bioimplant to bone is thatthe bone grows on and/or into the surface of the bioimplant. Variousstudies disclose that a calcium phosphate coating on an implant made ofcobalt-chromium (Co—Cr) or a titanium (Ti) alloy, for example, abiologic apatite accelerates bone adhesion more quickly than if theimplant made of the alloy has a non-coated surface. The biologic apatiteCa₁₀(PO₄)₆(OH)₂ is one of the main compounds which constitute human boneand teeth. The synthesized hydroxyapatite (HA) closely resembles anatural apatite and thus has been used in a study in which HA is used indental and orthopedic implants. An implant has been produced which iseasily integrated with neighboring bones and tissues by coating with HAor other crystalline calcium phosphates after transplantation.

However, although use of the synthetic joints in orthopedics fortreatment of degenerative joint diseases is a therapeutic treatmenteffective for reconstruction of joint function, microbes may proliferateon the surface of the synthetic joints, causing post-operativeinfection. It is because microbes can adhere to the surface of thesynthetic joint and the adhered microbes can form a habitat calledbiofilm. In such a case, antimicrobial agents (antibiotics) are noteffective any more, making it difficult to treat the infection. Moreoverif myelitis occurs, it is necessary to remove the synthetic joint andrepeat the surgery and there may be possibly a case where the infectedlimb should be ablated.

However, although use of the synthetic joints in orthopedics fortreatment of degenerative joint diseases is a therapeutic treatmenteffective for reconstruction of joint function, microbes may proliferateon the surface of the synthetic joints, causing post-operativeinfection. It is because microbes can adhere to the surface of thesynthetic joint and the adhered microbes can form a habitat calledbiofilm. In such a case, antimicrobial agents (antibiotics) are noteffective any more, making it difficult to treat the infection. Moreoverif myelitis occurs, it is necessary to remove the synthetic joint andrepeat the surgery and there may be possibly a case where the infectedlimb should be ablated.

Therefore, there are proposed a method of coating a hydroxyapatite filmhaving high crystallinity and large specific surface area, which issuited for impregnation with an antibiotic, by precipitatinghydroxyapatite on the surface of an implant and drying thehydroxyapatite, and a therapeutic agent-impregnated implant in which thecoating film is impregnated with the antibiotic (Patent Document 1). Thebioimplant prepared by the method is suited for impregnation ofantibiotics. However, since the coating film has uniform pore size andporosity, it is difficult to perform sustained release of a medicine ata desired rate and thus the medicine tends to be eluted at a fixed rateat a time.

Alternatively, the applicant proposed a method of controlling the rateof releasing an antibacterial or antimicrobial agent by adjusting theevanescence speed of HA by adjusting the crystallinity of the coatinglayer of calcium phosphate-based material (Patent Document 2)

PATENT DOCUMENTS

[Patent Document 1] JP-A No. 2005-506879; [Patent Document 2] JP-A No.2008-73098

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an scanning electron microscope (SEM) image of a surfaceof the spray coating without carrying out the hydration-treatment, andFIG. 1B shows an SEM image of the surface of a spray coating aftercarrying out the hydration-treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Technical Problems tobe Solved

As described in the above, one of the reasons of post-operativeinfection is that the microbes form the biofilm on the surface of theimplant and this causes the antimicrobial agents to be not effective.Especially, for 24 hours after operation, a risk of the microbialinfection is significantly high, because the immune function of apatient is significantly weakened. After that period, the patient slowlyrecovers own immune function. However, a highly possible state ofmicrobial infection continues over a long period ranging from one weekto several weeks after an operation. Especially, a patient havingautoimmune disease such as diabetes and further having a weak resistanceto the microbial infection (compromised host) has a high risk ofmicrobial infection. Further, a patient who develops infection afterimplant operation has high infection rates ranging from several times toseveral tens times in comparison with normal case, when having anoperation of replacing implant. Thus, a bioimplant which is capable toinhibit the biofilm formation over a long period of time after anoperation is needed. However, it was difficult for the conventionalbioimplant to be capable to inhibit the biofilm formation over such along period of time.

Thus, an object of the present invention is to provide a bioimplantwhich is capable to inhibit the biofilm formation over a long period oftime after an operation.

Means to Solve the Problems

The bioimplant according to the present invention, which was made toovercome the problems above, is characterized in that it comprises abase material of metal, ceramic or plastic and a thermal spraying filmmade of a calcium phosphate-based material formed at least partiallythereon, the silver concentration in the thermal spraying film being0.05 wt % to 3.00 wt %.

In the present invention, the calcium phosphate-based material ispreferably a compound or a mixture of two or more compounds selectedfrom the group consisting of hydroxyapatite, α-tricalcium phosphate,β-tricalcium phosphate, and tetracalcium phosphate.

Effect of the Invention

Since the bioimplant of the present invention can inhibit the biofilmformation over a long period of time after an operation, a curativeeffect of antimicrobial agents on infection can be maintained, andthereby decreasing risk of post-operative infection.. Further, as usedherein, “long period of time” refers to a period in which the risk ofthe post-operative infection is high, and the period ranges from oneweek to several weeks after an operation.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, favorable embodiments of the present invention will bedescribed in detail.

The bioimplant according to the present invention is a bioimplant,comprising a base material of metal, ceramic, or plastic and a thermalspraying film made of a calcium phosphate-based material formed at leastpartially thereon, the silver concentration in the thermal spraying filmbeing 0.05 wt % to 3.00 wt %.

The bioimplant according to the present invention include metal,ceramic, and plastic implants, such as synthetic bones and fixationdevices used for treatment of diseases and injuries, synthetic jointsused for reconstruction of lost joint function, and synthetic toothroots used for reconstruction of teeth.

A metal, ceramic, or plastic material may be used as the base materialof the bioimplant. A stainless steel alloy, a cobalt-chromium alloy,titanium, a titanium alloy, alumina, zirconia or the like may be used asthe metal, but titanium and titanium alloys are preferable. The titaniumalloys for use include alloys' of titanium with at least one metalselected from aluminum, tin, zirconium, molybdenum, nickel, palladium,tantalum, niobium, vanadium, platinum and the like. Preferably, it isTi-6Al-4V alloy. Alternatively, the ceramics for use include, forexample, alumina, zirconia, composite alumina-zirconia ceramics and thelike. Yet alternatively, the plastics for use include, for example,polyethylenes, fluorine resins, epoxy resins, PEEK resins, Bakelite andthe like.

The calcium phosphate-based material for use may be a compound or amixture of two or more compounds selected from the group consisting ofhydroxyapatite, α-tricalcium phosphate, β-tricalcium phosphate, andtetracalcium phosphate. It is preferably hydroxyapatite.

(Production Method)

The thermal spraying methods used for forming a thermal spraying film ofa calcium phosphate-based material include flame spraying method,high-speed flame spraying method, plasma spraying method, and coldspraying method. For example in the case of the flame spraying method, afilm is formed on the surface of a base material by melting a thermalspraying material or bringing it close to the melting state by placingit in a gas flame generated with oxygen and a flammable gas and sprayingthe resulting thermal spraying material on the base material. In thecase of the normal flame spraying method, the thermal sprayingtemperature is about 2700° C. and the thermal spraying speed is Mach0.6. Under normal thermal spraying condition, for example, a thermalspraying powder can be fed with 100 psi dry air into a gas frame torchgenerated with 50 psi oxygen gas and 43 psi acetylene gas and theresulting powder can be thermally sprayed at a thermal spraying distanceof 60 to 100 mm.

The thickness of the thermal spraying film is 5 to 100 μm, preferably 20to 40 μm, since it is not possible to cover the thermal spraying areaentirely when the thickness is less than 5 μm and the adhesion strengthof the film declines because of the residual stress during thermalspraying when the thickness is more than 100

It is preferable to carry out a heat-treatment of the prepared thermalspraying film. The heat-treatment can increase the crystallinity of acalcium phosphate-based material, and thereby improving the stability ofthe film. The heat-treatment may be carried out at a temperature rangeof 400 to 1000° C. for 0.5 to 7 hours under the reduced pressure of 10⁻²Pa or less. It is preferable to carry out at a temperature range of 550to 850° C. for 1 to 5 hours.

Further, it is preferable to carry out a hydration-treatment of theprepared thermal spraying film after carrying out the heat-treatment.The hydration-treatment can convert oxyapatite to hydroxyapatite, andresults in fine crystals of calcium phosphate being formed (e.g.,separated out from the coating and/or deposited/precipitated) on thesurface of the coating, thereby stabilizing an elution property ofsilver ion. As used herein, “separated out” refers to a process in whichan insoluble crystalized material is exposed or separated, whereas asoluble material is dissolved. Also, as used herein,“deposited/precipitated” refers to a process in which new crystals aregenerated and accumulated on the surface of a coating. The formation offine crystals of calcium phosphate on the surface of the coatingstabilizes the elution property of silver ions because the fine crystalscover the surface of the coating, which reduces the rate of elution ofsilver ions. The hydration-treatment includes a step of adding watermolecule to material and may be carried out, for example, by immersingthe thermal spraying film in water at a temperature of 60-100° C. for10-60 minutes, in accordance with some embodiments. FIG. 1A shows anscanning electron microscope (SEM) image of a surface of the spraycoating without carrying out the hydration-treatment and FIG. 1B showsan SEM image of the surface of spray coating after carrying out thehydration-treatment. As shown in FIG. 1B, it is observed thatneedle-shaped crystals, plate-shaped crystals, and/or granular-shapedcrystals are formed on the surface of the coating. In some embodiments,the size of the crystals in length, width or thickness is in the rangeof approximately 0.01 to 2.00 micrometers (μm).

It is possible to control the silver concentration in the thermalspraying film by adjusting the amount of the raw silver material blendedto the thermal spraying material, i.e., the calcium phosphate-basedmaterial. The silver concentration in the thermal spraying film is 0.05wt % to 3.00 wt %, preferably 0.05 wt % to 2.50 wt %, more preferably0.05 wt % to 1.00 wt %, and more preferably 0.1 wt % to 1.00 wt %. It isbecause the antimicrobial action is not sufficient when the silverconcentration is less than 0.05 wt %. Alternatively when it is more than3.00 wt %, the implant may become toxic to tissues and organs in thebody. According to literature, use of a great amount of silver leads toArgylia disease (disease leading to graying in color of the entireskin), decrease of leucocytes, and damage to liver and kidney. Ourstudies also showed that there are deformation of cells and inhibitionof neonatal bone formation when the silver concentration is more than3.00 wt %.

An example of the bioimplant according to the present invention is asynthetic joint consisting of a stem which is a bone contact portioninserted into the bone and a neck unit formed on the top end of the stemfor fixation of bone head ball, wherein at least part of the bonecontact portion is covered with a thermal spraying film of a calciumphosphate-based material and the silver concentration in thethermal-spray film is 0.05 wt % to 3.00 wt %. The synthetic joint ispreferably made of titanium or a titanium alloy.

[Examples—Experiment 1 (Sample Preparation)]

Hydroxyapatite containing a particular amount of silver oxide wassprayed onto one side of a pure titanium plate with a size of 50 mm×50mm×2 mm by flame spraying method, to form a thermal-spray film having athickness of about 40 μm. The flame spraying was carried out byintroducing, with 100 psi dry air, the thermal spraying powder into agas frame torch generated with 50 psi oxygen gas and 43 psi acetylenegas and spraying the fused powder at a thermal spraying distance of 60to 100 mm.

(Measurement of Silver Concentration)

After sufficient drying at 100° C., each sample was weighed and thendissolved in a nitric acid solution (5 mL of nitric acid and 50 mL ofpurified water) while heating. The silver concentration in the film wasdetermined by measuring the silver concentration in the solutionquantitatively by ICP emission spectrophotometric analysis. Then, thesample after removal of the film by solubilization was driedsufficiently and weighed again, and the film weight was calculated fromthe difference in weight from the sample before solubilization. Thesilver concentration in film (wt %) was calculated by dividing theamount of silver in film by the weight of the film. The silverconcentration of the film in this experiment was 0.3 wt %.

(Test for Antimicrobial Activity)

As a test for antimicrobial activity, an evaluation of performance ofinhibiting biofilm formation was carried out. Methicillin-resistantStaphylococcus aureus was adhered to samples, and then the samples wasimmersed in a fetal bovine serum retained at a temperature of 37° C. andwas cultivated under flow condition by stirring at 60 rpm. After culturefor one week and two weeks, biofilm formed on the film was stained byfluorescent staining and biofilm coverage on the film was determined andmorphology thereof observed by fluorescence microscope. Further, thebiofilm coverage was determined from a surface area ratio of afluorescence emission area obtained by using image analysis software.

Experiment 2

A sample was prepared in a similar manner with experiment 1 except thatHydroxyapatite containing no silver oxide was used, and supplied to thetest for antimicrobial activity.

(Result)

Table 1 shows the biofilm coverage. The biofilm coverage of the samplesof one-week culture and two-week culture in the experiment 1 becamelower than those of the control samples, indicating that the samples ofthe experiment 1 shows inhibiting the biofilm formation.

TABLE 1 Biofilm coverage (%) After culture for 1 week After culture for2 weeks Experiment 1 1.3 29.9 Experiment 2 5.7 48.9

Experiment 3

This experiment was carried out to reconfirm the effect of the silverconcentration in the film, and the samples having silver concentrationsof 0, 0.05, 0.1, 0.3 wt % were prepared.

(Sample Preparation)

Hydroxyapatite containing a particular amount of silver oxide wassprayed onto one side of a pure titanium plate with a diameter of 14 mmand a thickness of 1 mm by flame spraying method, to form athermal-spray film having a thickness of about 40 μm. The samples havingsilver concentrations of 0, 0.05, 0.1, 0.3 wt % were prepared byadjusting the amount of the silver oxide blended to the thermal sprayingmaterial,

The flame spraying was carried out in a similar manner as described inExperiment 1.

(Measurement of Silver Concentration)

The measurement of silver concentration was carried out in a similarmanner as described in Experiment 1.

Test for Antimicrobial Activity)

As a test for antimicrobial activity, a test of inhibiting biofilmformation was carried out. The test was carried out in a similar manneras described in Experiment 1 except that a culture medium was replacedevery fourth day to avoid the culture medium of fetal bovine serum frombeing saturated with a bacteria.

(Result)

Table 2 shows the results of measurements of the biofilm coverage. Thebiofilm coverage of the samples of one-week culture and two-week culturein this experiment became lower in proportion to the silverconcentration in the film. This indicated that biofilm formation isinhibited as the silver concentration in the film increases. Further,considering the results of this experiment and empirical knowledge onthe bacterial growth, it is estimated that the samples maintain aneffect of inhibiting biofilm formation for a long period time, such asabout four weeks when the silver concentration is 0.05%, about six weekswhen the silver concentration is 0.1%, and about ten weeks when thesilver concentration is 0.3%.

TABLE 2 Silver Biofilm coverage (%) concentration After one-week Aftertwo-week in film (wt %) culture culture 0 28.6 55.0 0.05 7.5 37.1 0.15.3 21.4 0.3 3.6 9.3

Since the bioimplant of the present invention is capable to inhibit thebiofilm formation over a long period of time after an operation, therisk of post-operative infection can be significantly decreased.Especially, a large effect can be expected when applying to patientshaving high risk of infection, for example, to a compromised host or apatient who develops infection after implant operation and have anoperation of replacing implant.

The foregoing outlines features of several exemplary embodiments of theinvention so that those of ordinary skill in the art may betterunderstand the aspects of the invention. Those skilled in the art willappreciate that they may readily use the present disclosure to makevarious changes, substitutions, and alterations to the embodimentsdisclosed herein to arrive at equivalent structures that are encompassedwithin the scope of the present invention. Thus, the scope of theinvention should not be limited by the exemplary embodiments disclosedherein but rather the scope of the invention should be commensurate withthe plain meaning of the claims issued from the present disclosure.

What is claimed is:
 1. A method of manufacturing a bioimplant,comprising: providing a base material of the bioimplant; applying athermal sprayed film of a calcium phosphate-based material to form acoating on at least a part of a surface of the base material; immersingthe coating in water at a predetermined temperature for a predeterminedperiod of time to convert oxyapatite present in the coating tohydroxyapatite and the resulting coating comprises a silverconcentration in the range of 0.05 wt % to 3.00 wt %.
 2. The methodaccording to claim 1, wherein said calcium phosphate-based material is acompound or a mixture of two or more compounds selected from the groupconsisting of hydroxyapatite, .alpha.-tricalcium phosphate,.beta.-tricalcium phosphate, and tetracalcium phosphate.
 3. The methodaccording to claim 1, wherein the base material comprises a metal. 4.The method according to claim 1, wherein the base material comprisesceramic.
 5. The method according to claim 1, wherein the base materialcomprises a plastic.
 6. The method according to claim 1, wherein finecrystals of calcium phosphate are formed on a surface of the coating asa result of the immersing, thereby reducing a rate of elution of silverions from the coating.
 7. The method according to claim 6, wherein asize of fine crystals of calcium phosphate is in a range of 0.01 μm to2.00 μm in length, thickness or width.